Connecting a Power Supply to a Speaker

[petemoore]

  I can't afford a scope, but know 50hz or 60hz can be made audible through a speaker, so it seems feasible I could listen for ripple on DC rails.
  What would be the simplest, most reliable circuit to make an audible monitor for my power supplies ?
  The input would need variable attenuation of some kind [a great amount of it for high voltage testing].
  The transducer would be 8ohms.
  If I knew how to block DC perfectly I wouldn't need to listen or look for it [can't afford a scope].
 

[Rixen]

A signal tracer:

Basically a low power amplifier and a speaker. I have one made with an LM386 amplifier IC, an attenuator pot and a capacitor to block DC.

I used to use a crystal earpiece, as these are very sensitive and high impedence. Probably a bit hazardous to have something stuck in your ear canal unless you are sure of the signal level, though.

With experience you can hear stuff with these devices you won't be able to see on a scope..

[SISKO]

  If I knew how to block DC perfectly I wouldn't need to listen or look for it [can't afford a scope].

A cap in series
Two cap in series.
A xfrormer

[printer2]

How about setting a meter to AC and measuring the voltage on the dc?

[TejfolvonDanone]

How about setting a meter to AC and measuring the voltage on the dc?

You get only RMS value. And even that isn't really RMS because a lot of meters only measure absolute mean value times 1.11. Which gives the correct result for only sine waves.
Ripple on DC isn't a sine and you get a lot more information (e.g. frequency) with a scope (or from hearing it) other than RMS value.

[antonis]

If I knew how to block DC perfectly I wouldn't need to listen or look for it

Don't get me wrong but if you don't know how to block DC it's aimless to hear (or look for) it...

This should only be helpful for exclusion purpose in debugging..

[petemoore]

  To sudio monitor what an Oscope input sounds like when monitoring power rails of a B+ high voltage.
  How to convert B+ signal voltage to an impedance and voltage capable of  driving an 8ohm load or audio input.
  To 'audio monitor' a B+ signal that I would otherwise be plugged into the 0-scope [I don't have].
  Perhaps a voltage divider...
  Or a power mosfet drain to B+ rail, Gate barely turned on, a small voltage [somehow derived at] across it's source resistor is output to speaker.

[Transmogrifox]

Basically a low power amplifier and a speaker. I have one made with an LM386 amplifier IC, an attenuator pot and a capacitor to block DC.
...
With experience you can hear stuff with these devices you won't be able to see on a scope..

If you're listening in on something like power amp rails and just want to know if the ripple will be audible in a speaker, then connect speaker in series with a 470 uF cap and connect that across your power supply rails.  If you care anything for your speaker, short the speaker while you connect the cap to the power rails (pay attention to polarity).  Then after a few seconds remove the short so the AC part of the ripple is driving the speaker.  That will keep the cap charging current out of your speaker.  Cap rating should be more than power supply rails you're measuring.

Or if you want to listen in on whatever is there whether it has enough power to drive a speaker or not (for example preamp power rails):  quick 'n' dirty connect it to the input of your guitar amp.  Use something like a 400V rated 0.22 uF cap in series with, say, a 10k resistor coming off the power supply rail you want to listen on.  Connect back-to-back 4.7 V zeners on your guitar amp side for protection.  The 10k + zeners will suppress the pop when you touch your "probe" to the power supply rail.  Again, good practice would be to short across the zeners while connecting the probe to the power supply so you don't send the pop through your amp.  The additional zener + resistor protection is to guard in case of accidents.

In reality you could use back-back diodes if you only want to listen for the presence of the noise. This will keep levels from getting out of control.  On the flip side leaving more headroom allows you to find out if your power supply filtering changes are doing something -- use the volume control to keep it from damaging your ears.  If it's clipped on back-to-back diodes, you improve filtering, you might not hear the difference even though it reduced ripple by 50% or something.  If the ripple is more than 4.7 volts on ANY rail, it's bad for audio purposes.

For example 1 Vpp may not be too bad on a 400V B+ feeding the power amp stage.  This is pretty well rejected by the power tubes high impedance.

Another way you can use the cap + resistor + zener/diode "probe" is to plug it into the sound card on your computer.  Then you can use a free software like Audacity to record a couple seconds of it and see what it looks like.  The PC sound card is the poor man's audio scope.

Here you might want to put the volume control into the "probe" in the form of a 500k pot followed by back-back LED's or so.  Here you would be interested in looking at the shape and not so much the amplitude.

However the sound card can be used to measure the amplitude if you input a clipped signal, which is clipped at a known level.  Input clipped signal into left, and measurement channel into right channel.  Then you can ascertain the amplitude by relating the test signal to the clipped signal peak.

[vigilante397]

I know you said you can't afford one, but I'm a big oscilloscope advocate. Sometimes they can be had for reasonable prices (~$100-150 used) and they tell you soooooo much more than other devices can. Sorry 

[garcho]

watching audio on an oscilloscope can be a blackhole for your noodle/breadboard-time, it's like a fire or a tv in a bar, so hard to take your eyes away! i've been messing with some lo-fi holtek/567 FSU circuits for drum machines and such, makes for awesome oscilloscope visuals.

[printer2]

How about setting a meter to AC and measuring the voltage on the dc?

You get only RMS value. And even that isn't really RMS because a lot of meters only measure absolute mean value times 1.11. Which gives the correct result for only sine waves.
Ripple on DC isn't a sine and you get a lot more information (e.g. frequency) with a scope (or from hearing it) other than RMS value.

While it is good to know what the actual voltages are what is really important is the difference, is it getting better with your changes. We are talking about the power supply here, most noise is in the range of a voltmeter.

Was going to add to use a little tube amp as a tester but Transmogrifox already mentioned it. I have a Champ type amp on my bench for the odd time I want to try something where I want to hear it but not be concerned for the safety of the amp. Mind you I have a scope so it doesn't happen too often.

[Rixen]

I've got one of these on the way:

http://www.surplustronics.co.nz/products/7009-pcb-scope-oscilloscope



300k/s
+/-20v
2 channel
$NZD40 (USD ~30 ?)

When I've checked it out, I'll write up a review.

cheers,
Julian
www.rixenpedals.com

[petemoore]

  Thanks for the feedback/suggestions!
  One of those meters would be useful.
  No problem to listen to rail noise now.
  Another affordable 'card type' measuring tool I want is for measuring capacitors and inductors.